Since titanium alloy is good in terms of the specific strength and corrosion resistance, it has been used in the fields such as aviation, military, space, deep-sea survey, and chemical plants. Recently, β alloy and the like have been attracting attention, and the usage fields of titanium alloy are about to further expand. For example, titanium alloys which exhibit a low young's modulus are about to be used for products adaptable to living bodies (for instance, artificial bones, etc.), accessories (for example, frames of eyeglasses, etc.), sporting goods (for instance, golf clubs, etc.), springs, and so forth.
Nevertheless, for the purpose of furthermore expanding the utilization of titanium alloys, it is indispensable after all to strengthen them. The mechanical characteristics of titanium alloys, such as the strength, are influenced greatly by the contents of interstitial (solid solution) elements like oxygen (O), nitrogen (N) and carbon (C). For example, when O solves in titanium alloys, it has been well known that their strength is improved. However, previous titanium alloys have been such that their ductility is impaired remarkably while their strength is improved.
Accordingly, in conventional titanium alloys, the admissible contents of interstitial elements such as O have been strictly regulated to predetermined values or less. For example, according to the ASTM (American Society for Testing and Materials) standard, in the case of pure titanium, it is classified as from type 1 to type 4 by the O contents. And, even in type 4 whose O content is the greatest, the content is limited to 1.2 at % (0.4% by mass) or less at the highest.
The circumstance is the same in commercially available titanium alloys as well. For instance, in the Ti-6Al-4V alloy (% by mass) being a multi-purpose α-β alloy, O is limited to 0.6 at % (0.2% by mass) or less, and N is limited to 0.1 at % (0.03% by mass) or less. Moreover, in the Ti-10V-2Fe-3Al alloy being a β alloy, O is limited to 0.5 at % (0.16% by mass) or less, and N is limited to 0.17 at % (0.05% by mass) or less. In addition, in the Ti-3Al-8V-6Cr-4Mo-4Zr alloy being β-C alloy, O is limited to 0.4 at % (0.12% by mass) or less, and N is limited to 0.11 at % (0.03% by mass) or less.
Thus, previous titanium alloys and pure titanium have been such that the contents of interstitial elements such as O are reduced extremely less, and that, even if they are set greater, they are only about 1.2 at % at the highest. Conventional titanium alloys have been such that the balance between the strength and ductility, which are in a trade-off relationship, is established by such an arrangement, however, the strength and ductility have been still insufficient so far so that it has not been possible to furthermore expand the utilization of titanium alloys.